Optical sensor

This sensor plugs into your FMM and,
using reflected infrared light, will signal your Apex when it changes state
from being wet to dry or dry to wet. This sensor can be used for constructing
your own ATO (auto-top-off), as a high-level warning sensor in your sump, water
storage level sensor, and much more. Included are a fastening nut and silicone
washer. These sensors are sold individually.

Also
available with an Optical Sensor Magnetic Mount.